U.S. Pat. No. 5,183,581, of the Applicant and herein fully incorporated by reference, teaches the use of a Nitrogen Generating System/Emulsion useful for the dewaxing of producing formations, whereby an increase in temperature provided by the reaction of nitrogen salts combined in the presence of an organic solvent in the external phase of the emulsion to cause paraffin removal from the reservoir. The salts used are an oxidizing nitrogen salt and a reducing nitrogen salt.
According to U.S. Pat. No. 5,639,313, also of the Appliant and herein fully incorporated by reference and directed to the dewaxing of hydrocarbon conduits, aqueous solutions which contain selected nitrogen reactants are individually prepared in mixing tanks and then added, still individually, to adequate volumes of a hydrocarbon organic solvent designed to efffect the dissolution of the specific kind of paraffin deposit found in the conduit or line. Also, dispersing agents and/or pour point depressants may be advantageously added to the thus prepared emulsions. The activation of the chemical reaction between the nitrogen reagents is effected with the aid of a solution of a weak organic acid such as acetic add. The emulsion contains in its internal phase the nitrogen salt which is stable in the slightly acidic medium having a pH between 3 and 6. The external phase is made up of an organic solvent or mixtures of organic solvents, especially hydrocarbon solvents. The breaking of the emulsion triggers the reaction of heat and nitrogen generation, which is called Nitrogen Generating System or SGN after the original Portuguese.
On the basis of the results of initial effective internal volume assessment of the conduit to be dewaxed, adequate volumes of the two emulsions are prepared which are simultaneously injected to the interior of the conduit, co-currently to the production flow by means of surface pumping systems. Support vessels comprise the terminals for injecting treating fluid at the inlet and for recovering the mixture of spent fluid and fluidized paraffin at the outlet.
The activation of the treating fluid is effected by means of acetic acid. The thus prepared fluid is then pumped at maximum possible flowrate from the vessel installed upstream of the conduit being treated. The mixture of emulsions C (ammonium chloride) and N (sodium nitrite) produces, exclusively on flow, the Nitrogen Generating System, emulsified SGN The emulsions are pumped at equal and constant flowrates while being displaced with the aid of a small bed of kerosene and seawater so as to place the fluid in the second half of the conduit. After the period of time that the fluid is left at rest so that it can effect the dissolution and removal of the paraffin deposit, the simultaneous pumping of emulsions C and N is resumed in order to treat the first half of the conduit. After another rest period the SGN treating fluid is withdrawn and the final internal effective volume is assessed in order to evaluate the efficacy of the treatment. Thus, the treating fluid using the Nitrogen Generating System works from downstream to upstream in the conduit to be dewaxed, the removal of the paraffin deposits being practically complete. However, as already stated before, the activation with acetic acid limits the use of this system to conduits of up to 4000 meters length and 4 inches internal diameter since the release of hydrogen ion is relatively quick and the emulsion is rapidly broken, and the nitrogen and heat generation reaction is initiated.
In spite of the excellent results in terms of paraffin removal obtained in the field by the technique set forth in U.S. Pat. No. 5,639,313, there are some drawbacks in the process which stem on the one hand from the relatively short time to the onset of the heat and nitrogen generating reaction which limits the length and diameter of the line to be dewaxed, and on the other hand, the high cost which derives from the use of support vessels where treating fluids are prepared and from where same are pumped to the line, this item representing nearly 70% of the overall cost of the process.
Thus, there was a need to develop a method able to effect the thermo-chemical dewaxing of highly-extended, high-volume hydrocarbon transmission conduits, and wherein the pumping of the treating fluid made up of one single aqueous solution of nitrogen salts could be effected from the production platform or from any equivalent equipment, this representing great savings. The pumping of one single solution which contained both the nitrogen salts, ammonium chloride and sodium nitrite, represents on its own a significant progress since this pumping should be done at constant flowrate, the feature of being constant being a difficult achievement when two solutions must be separately pumped. Also, because of the highly-extended feature of the conduit, the onset of the reaction of heat and nitrogen generation should be strongly delayed. Therefore, as described and claimed in U.S. application Ser. No. 08/742,126, the contents of which are fully incorporated in the present application, the Applicant has developed a polyanhydride-based polymer matrix designed to have an extended delayed action in the activation of the chemical reaction of the Nitrogen Generation System as applied to the dewaxing of large dimension lines.
However, as the polyanhydride of U.S. Ser. No. 08/742,126 was used in suspension in an aliphatic solvent, and in view of the large volumes of several cubic meters required to activate the SGN reaction as applied in the field, the pumping of the polyanhydride was rendered difficult and inaccurate, due to inhomogeneities or sedimentation of the suspended product which eventually led to variable hydrolysis rates of the polyanhydride. The applicant has thus developed new linear, aliphatic copolyanhydrides which are used in solution in a polar organic solvent, this leading to more uniform, accurate and easier pumping of the activator besides a better control of the hydrolysis rate of the delayed-action activator. The use of such copolyanhydrides as delayed-action activators constitute the improvement in the method for the thermo-chemical dewaxing of large dimension conduits described and claimed in the present invention.